Removable data storage units with flash memories, for personal computers and electronic appliances packaged very compactly—MMC memory card (the acronym standing for the expression “Multimedia Memory Card”) or USB key of “Mass Storage” type—are increasingly widespread. Initially envisaged for connecting to personal computers via input-output ports of USB type (the acronym standing for the expression “Universal Serial Bus”) or the like, these removable data storage units originally required the presence of a specific driver in the host station. As this hindered their portability, generic drivers have been rapidly developed, targeted at the main operating systems such as Windows®, Mac®, Unix®, Linux®, etc., operating on existing personal computers.
These generic drivers for managing removable data storage units such as, for example, drivers adapted for the communication protocol of the “USB mass storage device class” type, use specific protocols, endowed with a limited command set, for transactions between host station and removable data storage unit. Thus, according to the MMC-standard protocol, a removable data storage unit having a MMC card communicates with a host station via a limited set of standard commands intended for managing its memory space, including commands for reading and writing data.
Very rapidly, the need was felt to overcome the limitations of the command sets of these generic drivers by using the data exchanged in the course of a transaction initiated by a standard read/write command to pass additional instructions to the removable storage units from a host station.
Thus, Japanese patent application JP 2001-147850 proposes a scheme for authorizing or prohibiting the writing and/or reading of the memory space or of parts of the memory space or even only of files of a removable data storage unit without transmitting specific commands other than those authorized by a generic driver for managing a removable data storage unit. In this scheme, read/write authorization/prohibition state flags are associated with the physical addresses stored in the file allocation table FAT. A password precedes the data placed at the physical addresses forming the subject of a read/write restriction. In order to be considered, each read/write command in respect of physical addresses forming the subject of read/write restriction must be followed by a password alone for a read command or placed in the header of the data to be written for a write command. The password associated with a read/write command pertaining to a memory space or a file constrained by a read/write restriction must correspond to the password registered in order for the command to be executed.
It has also been proposed, in European patent application EP 1.571.557, that commands and/or data be passed to a removable data storage device by way of a standard write command originating from the generic driver of a personal computer operating system, by introducing into the data to be written a command identification flag detected and interpreted by the microcontroller managing the removable storage device as marking the presence of a command in the received data. Each data packet dispatched for writing to the removable storage device begins with a header field which may or may not contain the command identification flag and which is analyzed by the microcontroller managing the removable storage device prior to consideration of the data.
The passing of a command presence signaling or of a password by way of the data to be written in the removable data storage device exhibits the drawback of permitting fortuitous reproduction of command signaling or of a password by the ordinary data to be written that could lead the microcontroller of the removable device to an erroneous interpretation of write data as commands or a password.
The aim of the present invention is to substantially remedy the aforesaid drawback.